LUBRICANT OIL DEGRADATION SENSOR, SPEED REDUCER FOR INDUSTRIAL ROBOT, AND INDUSTRIAL ROBOT

A lubricant deterioration sensor, which is installed in a machine and detects deterioration of lubricant of the machine, is provided. A gap forming member forming an oil gap in which the lubricant enters, is provided. A supporting member, which supports a light emitting element, a color light reception element and the gap forming member, is provided. A fixing member to be fixed to the machine is provided. The gap forming member transmits light emitted from the light emitting element. The oil gap is disposed on an optical path from the light emitting element to the color light reception element. The fixing member supports the supporting member so as to be rotatable so that the direction of the opening of the oil gap changes when the supporting member rotates.